1. Field of the Invention
The present invention is a continuation in part of my U.S. patent applications Ser. Nos. 645,804; 754,035; 977,138 and 011,870 in which I disclosed methods for a boiler flue gas cleaning utilizing a step of cooling and condensing processes to remove its acidic water vapor and trace heavy metal vapors prior to separation of sulfur dioxide and carbon dioxide gases by thermodynamic equilibria and liquefaction processes. More particularly it relates to a reversible heat exchanger system where latent and sensible heat energy contained in the boiler flue gas stream is recovered and transferred to pre-heat a combustion air stream. Evaporative fluids circulating in closed heat exchanging circuits are employed to transfer heat energy between the relatively hot flue gas stream and the relatively cold combustion air stream.
A relatively hot flue gas stream flowing from a boiler at above 270.degree. F. is conducted to enter a flue gas cooling and condensing structure where the flue gas stream is cooled through several evaporator heat exchangers, as the flue gases flow on the outside of relatively colder extended metal surfaces of the evaporator heat exchanger, part of the water vapor contained in the flue gas condenses at temperature below the dew point of the flue gases.
While the flue gas acidic and hazardous vapors are condensed, a working evaporative fluid is heated and evaporated, then conducted to a corresponding condenser heat exchanger to give up the recovered heat energy to a combustion air stream; while the combustion air stream is heated, the saturated vapor of the working evaporative fluid is cooled and condensed then returned back to the evaporator heat exchanger.
2. Description of the Prior Art
In the present invention the latent heat capacity of a working evaporative fluid contained in at least one closed heat exchanging circuit is employed to effectively transfer and exchange heat energy between a relatively hot boiler flue gas stream and a relatively cold combustion air stream. Working evaporative fluids employed for the purpose of this invention are characterized to provide relatively low boiling and low condensing operating temperatures. The sensible and latent heat energy contained in the flue gas stream flowing on the outside surfaces of an evaporator heat exchanger is recovered to effect cooling the flue gases, and to condensing water vapor contained therein, while heating and evaporating the working fluid to a saturated vapor state. The saturated vapor of the working fluid is then conducted to give up heat to a combustion air stream in a condenser heat exchanger, while the combustion air is heated up to a relatively higher temperature, the vapor of the working fluid is condensed and recycled back to the flue gas cooling heat exchanger.
In the present invention the latent heat of evaporation of an evaporative working fluid provides larger heat transfer rate when compared to the prior art where lower heat transfer is limited by the heat exchanger metal conductance heat transfer capacity.
In the prior art, thermal storage rotating wheels, heat pipe heat exchangers, and plate type heat exchangers are used for combustion air preheating at higher dry temperature regions above the dew temperature point of the vapors contained in the flue gas stream. The present invention offers flue gas condensing heat exchanging system operating at much lower moist temperature regions, characterized to have high heat transfer rate, smaller heat exchanger surface area per unit of heat energy transferred, lower maintenance cost, lower capital cost, and economical cleaning of the heat exchanger elements. The present invention provides the flexibility to employ multiple evaporative heat transfer working fluids operating at adjustable pressures and temperatures to maximize the total heat transfer rate of the heat exchanging system.
In the condenser; the saturated vapor of a working fluid gives up heat to a combustion air stream and condenses over most of the heat exchanger length from a 100% vapor saturation state to 0% vapor. A temperature drop in a two phase flow results in a progressive pressure drop enhancing the vapor condensation rate. The thermal fluid condensate is further subcooled by the relatively colder combustion air, and the condensate is then pumped back to the evaporator side. In the evaporator; the evaporative working fluid is heated and evaporated to a saturation state. The evaporation occurs over most of the heat exchanger length from 100% liquid to 0% liquid state. A temperature increase in a two phase flow results in a progressive pressure increase to a point slightly above the saturation pressure and the 100% evaporation point before entering the condenser coil. Size and configuration of heat exchanger coils in each temperature zone will vary to satisfy the heat transfer rate, and the temperature differences between the flue gas stream and the evaporative working thermal fluid. Similar to the prior art; the evaporator and condenser heat exchangers may have multipath tubing arranged in serpentine fashion running perpendicular to the flue gas or combustion air flow. Tubes may be arranged in staggered rows to equilateral triangles, and parallel flow paths may be arranged to form regions of symmetry across the heat exchanger. Distributer headers connecting each circuit to ensure equal flow through the heat exchanger coils. The two phase in-tube evaporation and condensation process result in a higher heat transfer co-efficient and require a smaller heat transfer surface and higher exchange capacity.
Condensing furnaces are commercially available for residential and commercial air heating systems, the flue gas stream is cooled through a non corrosive aluminized steel heat exchanger where corrosive water vapor in the flue gas stream is condensed while its sensible and latent heat is recovered to pre-heat a recirculating room air stream. The present invention provides a high efficiency condensing heat exchanging system employing evaporative working fluids to cool a flue gas stream and to preheat a boiler combustion air stream, while cleaning the flue gas stream by condensing its acidic water vapor and removing other toxic and hazardous trace heavy metal vapors contained therein. The disclosed reverse heat exchanging system when employed for a boiler flue gas condensing and combustion air preheating will achieve a substantially higher heat recovery, cleaning the flue gas stream and reducing the heat rate of producing electric power. The recovery of more energy when increased excess air is required, and cleaning of more hazardous trace heavy metal vapors including dioxin from the flue gases emitting from incineration and bio-mass burning facilities will provide sound economics for the present invention.
The theory and practice of flue gas heat recovery heat pipe heat exchangers are well known for recovering and utilizing heat from hot flue gas stream in the dry gas temperature range between 500.degree. F. and 300.degree. F. to avoid condensation of acidic vapors, examples are disclosed in U.S. Pat. No. 4,616,697 by Kotaka, in foreign patent (Japan) document 60-2889 by Koutaka for a heat pipe heat exchanger device, and in document 61-217694 by Iwabuchi for a heat pipe type exchanger. However heat pipe heat exchangers are constructed from multiple sealed pipes, each contains evaporative fluid which evaporates at one end and condenses at the other end of the heat pipe. Heat pipes have internal wick and must be installed in tilted position from the horizontal to allow condensed fluid to flow by gravity to the evaporator end of the heat pipe. The working fluid normally disintegrate over a five to seven year period of time and require heat pipe replacement that become very expensive.
It is further acknowledged by the present applicant that there are some instances where it is known that heat exchangers were connected by pipe work for circulation of working fluid or refrigerant between them by pumping the condensed refrigerant from the cooler of the heat exchangers to the warmer heat exchanger; for example, in U.S. Pat. No. 4,476,922 by Heilig disclosed a heat exchanger system for recovering heat from an exhaust air stream for a work place application in commercial and industrial buildings, where the working place air temperature range is within the ambient temperature range. However this is an application in connection with a clean environmental air and the same objectives and problems of flue gas cleaning or preheating of boiler combustion air and feed water streams do not exist.
In U.S. Pat. Nos. 5,122,352 and 5,198,201 by Johnson disclose a method and apparatus for removing pollutants in a series of heat exchanger steps to cool the flue gas to above 200.degree. F. and to condense acidic vapor by indirect heat exchanger between the flue gas and boiler feed water. However this is an application in connection with extraction of sulphurous acid using chemical reaction between extracted sulfur dioxide gas stream and metallic iron bed to produce ferrous sulfite. The same objectives and the process of cleaning the flue gas by cooling to below ambient conditions and recovery of the flue gas heat energy to preheat the boiler combustion gas stream and boiler feed water using evaporative and condensing fluids heat exchangers as described in my application are not disclosed or discussed by the Johnson's process.
The use of temperature controls as described for this application is necessary for the proper operation of a high efficiency heat exchange system of this type. The theory and practice of temperature control systems has been used in the past in similar applications where a need to adjust the mass flow rates of a working heat transfer fluid that circulates between two heat exchangers to respond to deviations in the mass flow rate and temperature of an exhaust gas stream and incoming fresh air stream. Example of such system is disclosed by Hirayama in foreign patent (Japan) documents 61-252492 and 61-252493.